Two-Factor SSH Authentication with OTP, e.g. Google Authenticator
After copying two_factor_ssh to your /usr/bin/. Add the following line to your authorized_keys:
command="/usr/bin/two_factor_ssh 4rr7kc47sc5a2fgt" ssh-dsa AAA...zzz me@example.com
Code taken from: https://moocode.com/posts/5-simple-two-factor-ssh-authentication